Portable terminal apparatus for multimedia communication

ABSTRACT

The electric power consumption by a terminal used for communication of multimedia information is controlled by changing the quality of transmitted information. The terminal is provided with input (101, 102, 106 and 107) through which such information as images and sounds is inputted, channel control sections (123 and 124) which output the input information to channels and receive information from the channels, output (103, 104, 108, 109 and 105) which output the information received from the channels in the form of images, sounds, etc., a codec (110) which is provided between the input and output and the control sections, encodes the input information in one of multiple encoding modes in which electric power is differently consumed, and decodes the information inputted from the channels, and a control section (133) which controls the selection of the encoding mode. This terminal can continue information communication for a required period of time at minimum power consumption at the sacrifice of the quality of transmitted information. Therefore, either the power consumption or quality of information can be adequately selected according to the transmission.

TECHNICAL FIELD

The present invention relates to a multimedia communication terminalapparatus having a plurality of encoding modes with different powerconsumption. In the present invention, a change in the encoding modeindicates a change of a terminal state in which a transmitted encodingbit stream is changed even when the same input information source (e.g.,images inputted to a camera, sounds inputted to a microphone, etc.) issupplied to a terminal. Moreover, according to the present invention, aterminal for executing a communication of single information such assounds or the like also is included in the multimedia communicationterminal.

BACKGROUND ART

Multimedia communication terminals which can perform digital imagecommunication have been popularized really. As a main factor for puttingdigital image communication into a practice, there can be enumerated aprogress of image information compression technique (image codingtechnique) in addition to the development of electronic devicetechnique. Image encoding technique is a technique for compressing imageinformation having an information amount 1000 times as large as that ofaudio information to the level such that image information can betransmitted/stored. When a digital image communication using imageencoding techniques is carried out, a terminal on the transmission sideencodes (compresses), transforms inputted image information into anencoded bit stream, and transmits the encoded bit stream. A terminal onthe reception side obtains a decoded image by decoding a received bitstream. At that time, in order to correctly carry out the communication,terminals located on both ends of the channel have to use a common imageencoding system. To this end, as an international standard, there arenow determined standard image encoding systems such as H. 261, MPEG1 orMPEG2. It is expected that much more standard image encoding systemswill be determined from now on in accordance with new applications suchas a wireless image communication.

Most of image communication terminals that were already commerciallyavailable on the market are wired image communication terminalsconnected to an ISDN network line or the like. However, it is expectedthat wireless image communication terminals which can provide similarfunctions will become widespread in the future wireless networks. Thewireless communication terminal is featured in its portability, and itis customary that the wireless communication terminal uses a storagebattery as its electric power source. However, an amount of an electricpower that a light-weight storage battery used by the portable terminalcan supply is limited so that a power consumption of a terminal has tobe suppressed in order to prolong a continuous communication time. Also,since a surveillance system or the like uses a large number ofterminals, even when a storage battery is not in use, it is important tosuppress the power consumption. While the saving of the electric powerbecomes an important condition depending on applications as describedabove, an image encoding algorithm having a high information compressionratio has a large amount of calculations, and a power consumption tendsto increase. In general, it is a first object for the image encoding toincrease an information compression ratio, and a problem of a powerconsumption is not regarded as being important. However, in the futurein which terminals for multimedia communication will become widespreadand the problem of the power consumption that has been regarded as beingunimportant will hold an important position in the design anddevelopment of apparatus.

In order to save an electric power of commercially-available portableapparatus, notebook-type (laptop) computers had created some devices.Main power-saving functions of Macintosh Power Book 18C manufactured byApple Computer Inc., USA are as follows:

(a) When a battery remaining amount becomes lower than a constantamount, a backlight of a liquid-crystal display is automaticallydarkened;

(b) When a battery remaining amount becomes lower than a certainconstant level, an alarm message is displayed on a display;

(c) If a laptop computer, which is being energized, is left inoperativeduring a predetermined period of time, then it is automatically placedin a sleep mode (state in which all functions except for holding thememory contents of RAM are stopped);

(d) A user can switch a clock frequency of a CPU, and it becomespossible to save an electric power by lowering a processing speed; and

(e) When a CPU is not requested in processing during a predeterminedtime, a clock frequency of the CPU is lowered automatically. Such acountermeasure for decreasing a power consumption in the laptop computercauses a display screen to become difficult to see and also causes aprocessing speed to be lowered. However, in general, the content ofprocessing itself and provided information itself (e.g., informationitself to be displayed on the display screen) are not changed.

In general, since an encoding algorithm having a high informationcompression ratio executes a complex processing, a processing amount(calculation amount) tends to increase. As the calculation amountincreases, a power consumption also is caused to increase. For example,in a terminal for multimedia communication having a storage battery asan electric power source or a communication terminal in which a powerconsumption has to be decreased, a continuous communication time is as aresult reduced.

Therefore, an object of the present invention is to provide acommunication terminal in which a power consumption can be saved and anecessary continuous communication time can be maintained. Inparticular, it is an object of the present invention to provide acommunication terminal in which a power consumption can be decreased bychanging the encoding mode.

DISCLOSURE OF INVENTION

In order to attain the above-mentioned objects, in a communicationterminal according to the present invention, in image information oraudio information having a plurality of different encoding modes, apower consumption is controlled by switching the encoding mode into amode requested by a system or from the outside.

In a communication terminal apparatus in which inputted image or audioinformation is encoded and outputted to a channel and image or audioinformation obtained from the transmission line is decoded, a specificcommunication terminal apparatus according to the present inventioncomprises input means (101, 102, 106, 107) for inputting image or videoinformation, channel control sections (123, 124) for outputting theinputted information to a channel and inputting information from thechannel, output means (103, 104, 108, 109, 105) for outputtinginformation obtained from the channel by images, sounds or the like, acodec means (110) disposed among the input means, the output means andthe channel control sections for encoding the inputted information inaccordance with any encoding mode of a plurality of encoding modes withdifferent power consumption upon execution and decoding informationinputted from the channel and a control section (133) for controllingthe selection of the encoding modes.

In order to reduce the power consumption of the communication terminalapparatus much more, the quality of transmitted information, forexample, is controlled. It is provided with means for switching theencoding mode automatically or in a manual fashion in order to controlthe quality of the transmitted information. During a period in which acommunication is being made, it is possible for the communicated partyto switch the encoding mode. It is effective to provide with means formonitoring a battery remaining amount of a storage battery used for anelectric power source and means for presenting a standard of acontinuous communication possible time obtained when the mode is used atevery encoding mode to the person who executes the switching of theencoding mode and to switch the encoding mode at a proper timing. Also,it is provided with means for selecting an encoding mode with a smallpower consumption in accordance with a decrease of a detected batteryremaining amount. Further, it is effective to provide with means forstopping a clock signal to be supplied to circuits which becameunnecessary by switching the encoding mode. With respect to the clocksignal, it is effective to provide with means for much more lowering afrequency of a clock signal to be supplied to an encoding circuit as theprocessing amount of the operated encoding mode becomes smaller. It iseffective to provide with means for controlling a resolution when animage is transmitted and to control in such a manner as to have aresolution with a small power consumption.

A moving picture encoding will be described below by way of example. Theencoding mode includes a fixed intraframe encoding mode and anintraframe/interframe adaptive encoding mode and it is effective toreduce a power consumption to select the fixed intraframe encoding mode.Alternatively, in the intraframe/interframe adaptive encoding mode, itis effective to reduce a power consumption by increasing the frequencyof the intraframe adaptive encoding.

When the interframe encoding is carried out, it is effective to reduce apower consumption to provide means for executing a motion compensationbased on a block matching and reduce the number of searched motionvectors for every block in the block matching. Alternatively, it is alsoeffective to reduce a power consumption to provide means for controllinga frame rate of a transmitted image with respect to the moving pictureencoding and to reduce the frame rate.

When both of image and sound communication can be executed, if acommunication of only sounds is executed with a priority, then it ispossible to save an electric power and also to prevent data from beinglost abruptly.

In a communication terminal apparatus which is capable of transmittingand receiving color image information, to transmit and receive black andwhite image information is effective in reducing a power consumption.

When receiving an encoding mode switching request, the codec means mayswitch the encoding mode at the frame dividing point, the slice dividingpoint, or the block dividing point for block matching just after theencoding mode switching request.

If a power consumption is reduced when a battery remaining amountbecomes smaller, then a user is cautioned, and at the same time, aremaining conversation time can be prolonged. Also, if an optimumencoding system is selected based on a battery remaining amount and adesired conversation time, then it is possible to use an encoding systemwith a highest quality of picture in a range in which a conversationtime does not become shorter than the desired conversation time.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an example of a terminal formultimedia communication,

FIG. 2 is a block diagram showing an example of an H. 261 video encoder,

FIG. 3 is a block diagram showing an example of an H. 261 video decoder,

FIG. 4 is a block diagram showing an example of a software videoencoder,

FIG. 5 is a block diagram showing an example of a software videodecoder,

FIG. 6 is a block diagram showing an example of a multimediacommunication terminal of which the encoding mode is switched by aterminal user,

FIG. 7 is a block diagram showing an example of a multimediacommunication terminal of which the encoding mode is switchedautomatically,

FIG. 8 is a block diagram showing a multimedia communication terminal ofwhich the encoding mode is switched by those to whom communication ismade,

FIG. 9 is a diagram illustrative of the manner in which the encodingmodes will be changed in accordance with the remaining power amount ofthe battery,

FIG. 10 is a diagram illustrative of the manner in which an image isencoded when a frame rate is changed,

FIG. 11 is a block diagram showing an example of a circuit which puts aninterframe prediction section into the stop condition by ceasing thesupply of a clock signal, and

FIG. 12 is a block diagram showing an example of circuits disposedaround a CPU of a software codec which controls the power consumption byvarying a clock frequency.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a block diagram showing a fundamental example of a multimediacommunication terminal 100. This terminal has a function to carry out atwo-way communication concerning a moving picture, sounds and data, andsupplies an electric power to a circuit section 128 of a main unit by astorage battery 127. Battery power remaining information 132 detected bya battery remaining amount detector 131 is supplied to an encoding modecontrol section 133. This encoding mode control section 133 judges aproper encoding mode based on battery power remaining information or thelike, and supplies encoding mode control information 134 to avideo/audio codec 110. Also, similarly, a clock signal is supplied froma clock generating circuit 129 to the whole of the circuit unit of themain unit. An image signal is inputted to the multimedia terminal from acamera 101 and an audio signal is inputted to the multimedia terminalfrom a microphone 102, and the video and audio signals are converted byanalog/digital converters 106, 107 into digital signals, respectively.The digital signals are supplied to a video/audio codec circuit 110, andencoded by an encoding section 111. A video encoded bit stream obtainedfrom a video encoder 112 is supplied to a multiplexing circuit 121.Similarly, the audio signal is encoded by an audio encoder 115, andsupplied to the multiplexing circuit 121 as an audio encoded bit stream.The multiplexing circuit 121 further multiplexes three signals to whichthe output signal from the data input and output unit 105 is added. Amultiplexed signal output (output from the multiplexing circuit 121) isoutputted through a channel encoder 123 to the channel as a transmissionsignal 125.

On the other hand, a reception signal 126 from the channel is suppliedthrough a channel decoder 124 to a demultiplexing circuit 122, in whichit is separated into a video encoded bit stream, an audio encoded bitstream and a data signal. The audio and video encoded bit streams aresupplied to a decoder section 116. The video encoded bit stream isdecoded by a video decoder 117. The digital decoded image thus decodedherein is supplied through a digital/analog converter 108 and displayedby a display 103. Similarly, the audio encoded bit stream is decoded bythe audio decoder 120 as a digitized decoded audio signal, and outputtedfrom the loudspeaker 104 through a digital/analog converter 109. A datasignal 130 is supplied to the data input and output unit 105, and thenoutputted to a data processing apparatus such as a personal computer.

As an example of the video encoder 112 in FIG. 1, FIG. 2 illustrates anH. 261 encoder 200 which is an international standard of a communicationmoving picture encoding system. The H. 261 uses as the encoding system ahybrid encoding system (intraframe/interframe adaptive encoding system)which is a combination of a motion compensation prediction and a DCT(discrete cosine transform).

This hybrid encoding system will hereinafter be described with referenceto FIG. 2. A subtractor 202 calculates a difference between an inputtedimage (original image of present frame) 201 and an outputted image 213(which will be described later on) of an intraframe/interframe encodingchange-over switch 219, and then outputs an error image 203. This errorimage is transformed by a DCT transform unit 204 into a DCT coefficient,and quantized by a quantizer 205, thereby being outputted as a quantizedDCT coefficient 206. This quantized DCT coefficient 206 is outputted tothe channel as transmission information and also used within the encoderin such a manner that the interframe prediction processing unit 207synthesizes prediction images. The manner in which prediction images aresynthesized will be described next. The above-mentioned quantized DCTcoefficient 206 is processed by an inverse quantizer 208 and an inverseDCT transform unit 209 as a decoded error image 210 (the same image asthe error image reproduced on the reception side), and added with theoutput image 213(which will be described later on) from theintraframe/interframe encoding changeover switch 219 by an adder 211,thereby being outputted as a present frame decoded image 212 (the sameimage as the present frame image reproduced on the reception side). Thisimage is temporarily stored in a frame memory 214 and thereby delayed bya time of one frame. Accordingly, at the present time point, the framememory 214 outputs a preceding frame decoded image 215. This precedingframe decoded image and the present frame inputted image 201 areinputted to the motion compensation processing section 216, in whichthey are processed in an interframe prediction called a block matching.In the block matching, an image is separated into a plurality of blocksand the portion which is most similar to the original image of thepresent frame is extracted from the preceding frame decoded image atevery block, thereby a present frame prediction image 217 beingsynthesized. At that time, it is necessary to carry out a processing(motion estimation processing) for detecting how much each block ismoved between the preceding frame and the present frame. A motion vector220 of every block detected by the motion estimation processing istransmitted to the reception side. The reception side can synthesize thesame prediction image as that independently obtained on the transmissionside from this motion vector and the preceding frame decoded image. Theprediction image 217 is inputted to the intraframe/interframe encodingchange-over switch 219 with a "0" signal 218. This switch changes-overthe interframe encoding and the intraframe encoding by selecting eitherof the two inputs. When the prediction image 217 is selected (FIG. 2shows this case), there is carried out the interframe encoding. On theother hand, when the "0" signal is selected, the inputted image isDCT-encoded as it is and then outputted to the channel so that theintraframe encoding is executed. In order for the reception side tocorrectly obtain the decoded image, it is necessary to detect whetherthe interframe encoding or the intraframe encoding is carried out on thetransmission side. To this end, an identification flag 221 is outputtedto the channel. A final H. 261 encoded bit stream 223 is obtained bymultiplexing the quantized DCT coefficient, the motion vector andinformation of intraframe/interframe identification flag in themultiplexing circuit 222.

According to the H. 261, the intraframe/interframe encoding isindependently selected at every block. The encoder may freely select theintraframe/interframe encoding, and it is customary that the intraframeencoding or the interframe encoding with a higher coding efficiency isselected in response to the nature of the inputted image. In general,when a correlation between successive frames is low due to a scenechange or the like, it is said that a coding efficiency is made higherby selecting the intraframe coding.

FIG. 3 shows an example of a decoder 300 (corresponding to the decoder117 in FIG. 1) which receives the encoded bit stream outputted from theencoder of FIG. 2. A received H. 261 bit stream 317 is separated by ademultiplexing circuit 316 into a quantized DCT coefficient 301, amotion vector 302 and an intraframe/interframe identification flag 303.The quantized DCT coefficient 301 is processed by an inverse quantizer304 and an inverse DCT transform unit 305, and outputted as a decodederror image 306. This error image is added with an output image 315 ofthe intraframe/interframe encoding change-over switch 314 by an adder307, and then outputted as a decoded image 308. Theintraframe/interframe encoding change-over switch switches outputs inaccordance with the intraframe/interframe encoding identification flag303. A prediction image 312 used when the interframe encoding is carriedout is synthesized by a prediction image synthesizing section 311. Apreceding frame decoded image 310 stored in a frame memory 309 isprocessed in such a manner that the position is moved at every block inaccordance with the received motion vector 302. On the other hand, inthe case of the intraframe encoding, the intraframe/interframe encodingchange-over switch 314 outputs a "0" signal 313 as it is.

At present, it is frequently observed that the video encoder and decodershown in FIGS. 2 and 3 are realized by using special chips havingcircuits specialized in processing the respective units. On the otherhand, as a new form used instead of the conventional video encoding anddecoding apparatus using the special chips, an encoding/decodingapparatus (software codec) based on a software using a general-purposeCPU receives an attention recently. Since the software codec has lesstime and labor for developing apparatus and also has features to copewith variations of functions and performance with a flexibility, it isexpected that the number of software codecs will increase as ageneral-purpose CPU becomes higher in processing speed.

FIGS. 4 and 5 show examples of software encoder 400 and decoder 500 formoving picture, respectively. 400 and 500 correspond to 112 and 117 inFIG. 1, respectively. In the software encoder 400, an input image 401 isstored in an input frame memory 402, and a CPU 403 reads out informationtherefrom and encodes the same. A program for driving this CPU is storedin a program memory 404. Also, the CPU executes an encoding processingby effectively utilizing a processing memory 405. Encoding informationoutputted from the CPU is temporarily stored in an output buffer 406,and then outputted as an encoding bit stream 407.

On the other hand, in the software decoder 500, an inputted encoding bitstream 501 is temporarily stored in an input buffer 502 and read by aCPU 503. The CPU executes a decoding processing by effectively utilizinga program memory 504 and a processing memory 505. A resultant decodedimage is temporarily stored in an output frame memory 506, and thenoutputted as an outputted image 507.

Examples of portions in which the multimedia communication terminalshown in FIG. 1 controls the encoding mode in response to the remainingamount of electric power are illustrated in FIGS. 6, 7 and 8.Hereinafter, the same references in the sheets of the drawings identifythe same elements and parts.

In the arrangement shown in FIG. 6, a battery remaining amount detectingsection 131 detects a residual electric power amount of the battery 127,and transmits the detected residual electric power amount to an encodingmode control section 133 as battery remaining amount information 132.When this information 132 is information indicative of the fact that thebattery remaining amount of the battery 127 is lower than a certainconstant value, an alarm display control section 604 configuring theencoding mode control section 133 generates an alarm to a terminal user606. As a means for transmitting an alarm, there might be considered adisplay such as an alarm message, an alarm symbol on display forvisually appealing the terminal user and an alarm such as beep sounds orsynthesized sounds for appealing to the ear of the terminal user. Also,it is effective that the arrangement of FIG. 1 further includes an alarmlamp to flush to caution the terminal user. Thus, when the terminal userdetects that the battery remaining amount becomes small, the terminaluser is able to change the encoding mode by changing-over the switch.When receiving switching information 607, an encoding mode switchingsection 605 switches the encoding mode in accordance with an instructionfrom the terminal user. When the terminal user switches the encodingmode, with respect to each encoding mode, it is possible to displayauxiliary information such as continuous communication time which areexpected when a communication is continued in that mode. A specificmethod for switching the encoding mode will be described later on.

While the user switches the encoding mode in the terminal of FIG. 6, theterminal shown in FIG. 7 is characterized in that an encoding modeautomatic switching section 701 automatically switches the encoding modein response to the battery remaining amount information. At that time,the terminal user can freely program in advance the encoding mode inaccordance with the level of the battery remaining amount. Also, theterminal user can select a desired one from a plurality of patterns thatare programmed in advance. As an automatic control method, there mightbe generally considered a system in which the encoding mode is moved toa low power consumption/low quality mode as the battery remaining amountdecreases. Thus, there can be expected effects such as preventing acommunication from being suddenly stopped when a battery has run down.

FIG. 8 shows an example of a system in which an encoding mode of aterminal which transmits information is controlled by a terminal 802 ofthe called party through a channel 801. In a surveillance system using asurveillance camera or the like, there might be considered the case inwhich an encoding mode of a camera is controlled in a remote operationfashion. Also, if there are a number of surveillance cameras, then it ispossible to simplify the arrangement of each surveillance camera byintensively controlling the encoding mode with the terminal of thereception side.

The power consumption control systems based on the encoding modeswitching shown in FIGS. 6, 7, 8 can also be effective when a storagebattery is not used as an electric power source in particular. In thesurveillance system having a number of surveillance cameras, forexample, in order to suppress a total power consumption of allsurveillance cameras, there might be considered the system in which anencoding mode of a camera which does not transmit important information,in particular, is moved to a low quality/low power consumption mode.

A specific method of switching the encoding mode will be described next.It is assumed that the H. 261 encoder and decoder shown in FIGS. 2 and 3are used as the video encoder 112 and decoder 117 of the multimediacommunication terminal 100 shown in FIG. 1. For example, as the movingpicture encoding modes, the following four modes are prepared:

(1) Transmission and reception based on interframe/intraframe adaptiveencoding:

(2) Reception is the same as (1) and transmission has only theintraframe encoding:

(3) Reception is the same as (1) and transmission is not made; and

(4) Image is neither transmitted nor received.

In this case, when the encoding mode (1) is used, all portions in thevideo/audio codec circuit are operated. When the encoding mode isswitched from (1) to (2), the interframe prediction processing section207 becomes disabled. Then, when the encoding mode becomes (3), theremaining portions of the video encoder 200 become disabled. Finally,when the encoding mode becomes (4), further the video decoding section117 becomes disabled, and the transmission and reception of videoinformation are all stopped. As the number of the encoding modeincreases, the disabled portions become large so that the powerconsumption becomes small. However, on the contrary, as the encodingmode has small power consumption, the quality of transmitted informationis lowered. FIG. 9 shows an example of a relationship between theencoding mode and the battery remaining amount of this system. Referencenumeral 901 denotes a graph showing battery remaining amounts obtainedwhen the battery remaining amount (electric power×time) presented underthe condition that the battery is fully charged is set to 1.0 and thebattery remaining amount obtained when the battery remaining amountunder the condition that the battery becomes disabled is set to 0.0.When the battery remaining amount becomes less than 0.5, for example,the encoding mode is changed from (1) to (2). To simplify theprocessing, it is suitable that a timing at which the encoding mode isswitched is set to a first frame dividing point obtained immediatelyafter the battery remaining amount becomes lower than a predeterminedvalue. If it is intended to fine control the power consumption, thenthere might be a method in which this switching point is set to adividing point of blocks for block matching or a dividing point of aslice (column of blocks horizontally extended from end to end of imagefor block matching). In these dividing points, independent processingcan be carried out for the preceding and succeeding units for processing(frame, slice, block) so that the encoding mode can be switched withease. When the user switches the encoding mode, it is suitable to switchthe encoding mode from the dividing points of frame, block, slicepresented immediately after a similar switching request is issued.

In the example of FIG. 9, it is possible to further fine control thepower consumption by changing parameters concerning the encoding in therespective encoding modes. In the encoding mode (1), for example, theframe rate of the transmitted image should be lowered. Then, it becomespossible to relatively decrease a time in which the video encodingsection is operated. As a consequence, it is possible to decrease thepower consumption. FIG. 10 shows this state.

1001 shows the manner in which the encoding section is operated when theencoding is carried out at n frames/second. A portion shown hatchedindicates a time during which the encoding section is operated. On theother hand, 1002 shows the manner in which the encoding section isoperated when the encoding is carried out at n/2 frame/second. Since theprocessing of every frame is ended at 1/n second, the video encodingsection can be placed into the stop state during 1/n second at every 1/nsecond. A similar control can be carried out by changing the resolution(number of pixels) of an image.

Except the frame rate and the resolution, a frequency at which theintraframe encoding is selected in the interframe/intraframe adaptiveencoding can be used as a parameter. When the frequency at which theintraframe encoding is selected is raised, it becomes possible torelatively decrease the time during which the interframe predictionsection is driven. Thus, a power consumption can be decreased. Also, inorder to decrease a power consumption, it is effective to narrow amotion vector search range used when a block matching is carried out inthe interframe encoding. Further, when a terminal which transmits orreceives a color image transmits or receives a black and white image, acircuit or memory which handles color difference signals can bedisabled. Further, even when a bit stream in which a color image isencoded is received, similar effects can be achieved by reproducing onlyblack and white information. However, to lower the frame rate and theresolution, to raise the frequency at which the intraframe encoding isselected, and to narrow the searching range of the block matching alllead to the deterioration of an image decoded on the reception side.

The manner in which a control is made from a circuit standpoint when apower consumption of a disabled circuit is decreased will be describedbelow. As a direct method of preventing the disabled circuit fromconsuming an electric power, there is a method of stopping the supply ofan electric power by using a power transistor. Although this method iseffective, when the whole of the circuit is integrated, the powertransistor occupies a large area, resulting in a problem. Therefore,there is considered a method of stopping a clock signal. FIG. 11 showsan example of a circuit which switches the operating state of theinterframe prediction section by stopping the supply of the clocksignal. In the multimedia communication terminal 100 of FIG. 1, the H.261 encoder 200 shown in FIG. 2 is used as the video encoder 112. Inthis circuit, a clock signal 1101 generated by the clock signalgenerator 129 is supplied through an AND circuit 1103 to a clock input1104 of the interframe prediction processing section 207. An encodingmode switching signal 1102 is supplied from the encoding mode switchingsection 605 in FIGS. 6 and 8 or the encoding mode automatic switchingsection 701 in FIG. 7. when the encoding mode switching signal is set to"1", the clock signal reaches the interframe prediction section.Whereas, when the encoding mode switching signal is set to "0", theclock signal is not supplied. Since the clock signal is not supplied,the interframe prediction section is placed in the stop state. As aconsequence, it becomes possible to suppress the power consumption ofthis portion.

On the other hand, in the software codec, since most of the imageencoding and decoding is executed by the CPU, it is difficult to use amethod which controls a power consumption by stopping the disabledcircuit. Therefore, there is considered a method of controlling afrequency of a clock signal which is supplied to the CPU. To lower theframe rate, to raise the frequency of the intraframe encoding, to stopthe transmission of image signal and to stop the reception of imagesignal lead to the decrease of the processing amount of the CPU in thesoftware codec. Accordingly, even when the clock frequency is lowered bythe decreased processing amount, there can be maintained a real timeproperty of the processing. In general, it is known that a powerconsumption of a general-purpose CPU decreases as a frequency of anoperation clock is lowered. Therefore, to control the clock frequencyleads to the control of the power consumption. FIG. 12 shows an exampleof a clock control circuit in a software encoder which switches a CPUclock in response to the encoding mode. In the multimedia communicationterminal 100 of FIG. 1, there is used the software encoder 400 shown inFIG. 4 as the video encoder 112. An encoding mode switching signal 1102changes a state of an encoding mode command register 1209. content 1208of the encoding mode command register can be read out by a CPU 403 whena certain specific memory address is read out. As mentioned earlier,since the switching points at which the encoding modes are switched arelimited (the above-mentioned frame, slice and block dividing points),the CPU may read the content of the encoding mode command registerimmediately before the processing of the switching point is carried out.The CPU detects on the basis of the content of the encoding mode commandregister that the encoding mode switching command is issued, andswitches the encoding mode at a certain switching point. Incidentally,the encoding mode switching request processing for the CPU can beattained by interrupting the CPU. The CPU which switches the encodingmode writes a CPU encoding mode 1206 in a CPU encoding mode register1205 in order to announce the present encoding mode of the CPU itself toexternal circuits. A clock frequency-dividing section 1203 reads thecontent 1202 of the CPU encoding mode register, and controls afrequency-dividing ratio (ratio in which a frequency is lowered) of aclock signal 1101 generated by the clock signal generator 129 inresponse to the encoding mode of the present CPU. In this way, thefrequency of the clock signal 1204 supplied to the CPU is controlled inresponse to the encoding mode. Incidentally, the processing of thatportion can be realized by controlling the oscillation frequency itselfof the clock with a VCO (Voltage Controlled Oscillator).

The system for controlling the clock signal as described above can beapplied to also an apparatus using a special chip. When a frame rate islowered, for example, it is possible to decrease a power consumption bylowering an operation clock of the encoding apparatus itself withoutusing the system shown in FIG. 10.

In the present specification, the change in the same encoding system(e.g., H. 261) is handled as the change of the encoding mode. However,it is clear that to change the encoding system itself (for example, thechange from MPEG1 to H. 261) also is similarly included in the frameworkof the present invention as the change of the encoding mode.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to decrease theamount of electric power consumed by a terminal by intentionallylowering the quality of encoded information which is transmitted by aterminal. A communication terminal apparatus of the present invention isuseful for a portable communication terminal apparatus using a storagebattery, for example, a video surveillance apparatus whose main objectis an image transmission continuous time while suppressing a powerconsumption, etc.

We claim:
 1. In a communication terminal apparatus in which inputtedvideo or audio information is encoded and outputted to a channel andvideo or audio information obtained from said channel is decoded, saidcommunication terminal apparatus characterized by input means forinputting video or audio information, channel control sections foroutputting said inputted information and receiving information from saidchannel, output means for outputting information obtained from saidchannel in the form of image or sounds, a codec means disposed amongsaid input means, said output means and said channel control sectionsfor encoding said inputted information and decoding information inputtedfrom said channel in accordance with any encoding mode of a plurality ofencoding modes with different power consumption upon execution and acontrol section for controlling the selection of said encoding mode. 2.In a communication terminal apparatus as claimed in claim 1, saidcommunication terminal apparatus further comprising a supply means forsupplying electric power to the whole of the apparatus and in which saidcontrol section controls the selection of said encoding mode in responseto a battery remaining amount of said supply means.
 3. In acommunication terminal apparatus as claimed in claim 2, saidcommunication terminal apparatus characterized in that said controlsection outputs video or audio message by using said output means inresponse to a battery remaining amount of said supply means and controlsthe selection of said encoding mode in response to a control signalinputted to said message.
 4. A communication terminal apparatus asclaimed in claim 2, in which said supply means includes a storagebattery and a means for detecting a remaining battery amount of saidstorage battery, and said control section switches the encoding mode toa different encoding mode corresponding to a low power consumption ascompared with a power consumption required by a present encoding modewhen information indicative of a battery remaining amount smaller than apreviously-determined battery remaining amount is supplied to saidcontrol section.
 5. A communication terminal apparatus as claimed inclaim 3, in which said message includes a successive communicationpossible time in said encoding mode.
 6. A communication terminalapparatus as claimed in claim 1, characterized in that the supply of aclock of a whole apparatus to unnecessary circuits is stopped byselectively switching said encoding mode.
 7. A communication terminalapparatus as claimed in claim 1, in which a frequency of a clocksupplied to the whole of apparatus is changed in response to a processedamount of said encoding mode.
 8. In a communication terminal apparatusas claimed in claim 1, said communication terminal apparatus furthercomprising a supply means for supplying an electric power to the wholeof apparatus, and in which said control section lowers a resolution ofsaid inputted information when a battery amount of said supply meansbecomes smaller than a predetermined amount.
 9. In a communicationterminal apparatus as claimed in claim 2, said communication terminalapparatus characterized in that said plurality of encoding modes includea mode for carrying out a fixed intraframe encoding and a mode forcarrying out an intraframe/interframe adaptive encoding concerning amoving picture encoding.
 10. In a communication terminal apparatus asclaimed in claim 9, said communication terminal apparatus characterizedin that said control section selects said fixed intraframe encoding modeas said encoding mode in response to the decrease of the batteryremaining amount of said supply means.
 11. In a communication terminalapparatus as claimed in claim 9, said communication terminal apparatuscharacterized in that said control section increases a frequency atwhich an intraframe encoding in said intraframe/interframe adaptivecoding is selected in response to the decrease of the battery remainingamount of said supply means when the mode for carrying out saidintraframe/interframe adaptive encoding is selected as a presentencoding mode.
 12. In a communication terminal apparatus as claimed inclaim 2, said communication terminal apparatus characterized in thatsaid codec means includes a means for executing an interframe codingconcerning a moving picture encoding and a means for executing a motioncompensation based on a block matching, and said control section reducesthe number of searched motion vectors at every block in said blockmatching in response to the decrease of the battery remaining amount ofsaid supply means.
 13. In a communication terminal apparatus as claimedin claim 2, said communication terminal apparatus characterized in thatsaid codec means includes a means for executing an interframe codingconcerning a moving picture encoding and a means for controlling a framerate of an image to be transmitted, and said control section lowers saidframe rate in response to the decrease of the battery remaining amountof said supply means.
 14. In a communication terminal apparatus asclaimed in claim 2, said communication terminal apparatus characterizedin that said control section performs a communication only by sounds inresponse to the decrease of the battery remaining amount of said supplymeans.
 15. In a communication terminal apparatus as claimed in claim 2,said communication terminal apparatus characterized in that said controlsection transmits video information inputted from said input means inthe form of a black and white image in response to the decrease of thebattery remaining amount of said supply means.
 16. In a communicationterminal apparatus as claimed in claim 2, said communication terminalapparatus characterized in that said control section decodes imageinformation received from said channel as black and white imageinformation in response to the decrease of the battery remaining amountof said supply means.
 17. In a communication terminal apparatus asclaimed in claim 16, said communication terminal apparatus characterizedin that said control section reproduces a black and white image exceptcolor information from a bit stream which encodes a color image receivedfrom a channel.
 18. In a communication terminal apparatus as claimed inclaim 2, said communication terminal apparatus characterized in thatsaid codec means switches an encoding mode from a frame dividing pointprovided immediately after said codec means receives an encoding modeswitch request signal from said control section.
 19. In a communicationterminal apparatus as claimed in claim 2, said communication terminalapparatus characterized in that said codec means switches an encodingmode from a slice dividing point provided immediately after said codecmeans receives an encoding mode switch request signal from said controlsection.
 20. In a communication terminal apparatus as claimed in claim2, said communication terminal apparatus characterized in that saidcodec means switches an encoding mode from a dividing point of blocksfor block matching provided immediately after said codec means receivesan encoding mode switch request signal from said control section.